A fuel cell, which is a new electricity generation system using electrical energy directly converted from the energy produced by the electrochemical reaction of a fuel gas and an oxidant gas, is under careful examination for use as a power source, such as that for space stations, unmanned facilities at sea or along coastal areas, fixed or mobile radios, automobiles, household electrical appliances or portable electrical appliances.
Fuel cells are divided into a molten carbonate electrolyte fuel cell operating at a high temperature (in the range of about 500.degree. C. to about 700.degree. C.), a phosphoric acid electrolyte fuel cell operating around 200.degree. C., an alkaline electrolyte fuel cell operating at room temperature to about 100.degree. C., and a solid electrolyte fuel cell operating at a very high temperature (1,000.degree. C. or above).
A molten carbonate fuel cell (MCFC) is constituted by a porous nickel anode, a lithium-doped porous nickel-oxide cathode and a lithium aluminate matrix which is filled with lithium and potassium carbonate as electrolytes. The electrolytes are ionized by melting at about 500.degree. C., and the carbonate ions generated therefrom establish electron flow between the electrodes. Hydrogen is consumed in the anode area, which thus produces water, carbon dioxide and electrons. The electrons flow to the cathode via an external circuit to produce the desired current flow.
The matrix of an MCFC is generally composed of gamma-lithium aluminate (.gamma.-LiAlO.sub.2) to be formed as a porous tile of about 0.5-2 mm in thickness. Such a matrix supports a mixed carbonate (Li.sub.2 CO.sub.3 /K.sub.2 CO.sub.3), which is an electrolyte, and offers a path for carbonate ions (CO.sub.3.sup.-2) generated from the cathode to move toward the anode during operation. Further functions of the MCFC matrix are to provide electrical insulation between the electrodes, to prevent the mixing of various reactants such as fuels introduced to the respective electrodes or to the air, and to serve as a wet seal so that harmful gases do not leak from the cell. Differently from other electrochemical systems, the matrix in which electrolytes are impregnated exists as a solid at room temperatures and as a paste at 650.degree. C., which is the operating temperature of an MCFC. Therefore, the performance of a matrix is determined by internal resistance, gas cross-over, the wet seal function of fuel cells, etc.
An MCFC matrix is subject to great stresses due to the large temperature differential which exists between room temperature and the cell operating temperature, and thus, functional stability is a prevailing problem. Therefore, investigations for overcoming the problem of functional stability are being made. For example, U.S. Pat. No. 4,322,482 disclosed that when 20 volume percent or less of lithium aluminate particles and/or alumina particles whose diameters are at least 50 microns are added to submicron lithium aluminate support particles, the formation of cracks due to temperature change is prevented. Also, U.S. Pat. No. 4,511,636 disclosed a matrix which can withstand a compression force occurring at the time of manufacture and is thus not fragile, by mixing an inert material whose diameter is less than about one micron with ceramic particles having diameters greater than about 25 microns and then adding about 35 volume percent of organic binder thereto. Here, lithium aluminate and alumina are examples of the inert material and the ceramic particles, respectively.
Lithium aluminate exists in three phases (states), namely, alpha (.alpha.), beta (.beta.) and gamma (.gamma.). Of these phases, the .gamma.-phase is the most stable at MCFC operation temperature of 650.degree. C., and matrices composed of the .gamma.-lithium aluminate are the most widely employed.
FIG. 1 is a photograph taken by a scanning electron microscope (SEM), showing a pellet manufactured by sintering the pure .gamma.-lithium aluminate at 1,200.degree. C. for four hours according to a conventional method. Here, the matrix for the conventionally manufactured MCFC has relatively small (about 1 .mu.m) .gamma.-lithium aluminate particles. Therefore, the resultant matrix does not have adequate pore distribution and the intensity is low, since structural stability is not maintained. Also, due to a high sintering temperature used in manufacturing the matrix, the overall manufacturing cost is considerably high.